Hydrogen is the most abundant element in the Universe and very common on earth. Its atomic structure is the simplest of all atoms, since it is composed of one proton and one electron. Usually, pure hydrogen, which is diatomic (H2), does not exist naturally since it easily combines with other elements. Hydrogen is mainly chemically bound in water, biomass or fossil fuels. In order to get hydrogen into a useful form, it must be extracted from one of these substances. This process requires energy. Accordingly, the cleanliness and renewability of this energy is of critical importance. While hydrogen fuel cells operate without producing emissions, making hydrogen can produce significant greenhouse gases and other harmful byproducts. Nevertheless, once obtained, hydrogen is a nearly ideal energy carrier.
There exist various ways for hydrogen processing which are briefly described below although hydrogen from biomass is focused in this handbook:
Electrolysis: Water electrolysis involves passing an electric current through H2O to separate it into hydrogen (H2) and oxygen (O2). Hydrogen gas rises from the negative cathode and oxygen gas collects at the positive anode. Electrolysis produces extremely pure hydrogen, but a large amount of electricity is required. Ideally, this would come from renewable sources like wind and photovoltaic.
Steam-Methane Reformation: Hydrogen can be reformed from natural gas by a two-step process at temperatures reaching 1100 °C in the presence of a catalyst. This is a relatively efficient and inexpensive process, especially in case of cogeneration.
Photoelectrolysis: Photoelectrolysis uses sunlight to split water into its components via a semi-conducting material. It is roughly like immersing a photovoltaic cell in water, whereby the incoming light stimulates the semiconductor to split H2O directly into its constituent gases. Though promising, this is still an experimental method of hydrogen production that has not evolved beyond the laboratory.
Hydrogen from Coal: Coal contains hydrogen, and techniques are being developed to sequester hydrogen and carbon. Nevertheless, coal mining pollutes and despoils the landscape, and burning coal produces many harmful emissions.
Bio-Hydrogen: Certain species of green algae produce hydrogen in the presence of sunlight. Researchers manipulated the photosynthetic process of spinach plants to produce hydrogen. But these biological means of hydrogen production, like the photoelectrolytic process described above, are known only as immature lab experiments. Intense research persists to better understand ways to improve these hydrogen production methods.
Biohydrogen: Apart from the above mentioned technologies, the conversion route from biomass to hydrogen gains on interest, as it is a pathway based on renewable energy that can therefore contribute to the reduction of GHG. The conversion routes for hydrogen from biomass are therefore described below.
There are currently two possible production routes for biohydrogen, the gasification of solid biomass and the digestion of (usually water rich) biomass, both with following purifying and reforming of the produced syngas to hydrogen. For both conversion routes there is a strong competition to the direct use of biomass.
Using biomass gasification, hydrogen-rich biomass sources converts to synthesis gas when heated in a controlled atmosphere. This synthesis gas (see chapter 7.2.1) primarily consists of carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2). It is often composed of several compounds from which the H2 must be extracted afterwards. Gasification can be conducted using a variety of low, medium or high- temperature methods. These methods differ in several aspects, including required pre-treatment and postgasification treatment. Gasification technology has been under intensive development in recent times. Large- scale demonstration facilities have been tested and commercial units are starting worldwide. Nevertheless, the barriers for biomass gasification have been economic rather than technical. If biomass gasification to hydrogen is combined with carbon sequestration, biomass could even serve as a measure for returning CO2 from the atmosphere into the earth .The other method of gaining hydrogen from biomass is the digestion of biomass. In traditional biochemical conversion (digestion) processes, wet feedstock such as manure is digested to produce primarily CH4 and CO2 . In order to produce hydrogen, the CH4 has to be converted by using a thermochemical process, such as steam reforming. By manipulation of process conditions, methane formation can be suppressed and hydrogen can be directly produced along with organic acids. These acids can then be converted into methane and post-processed to yield additional hydrogen, increasing the overall efficiency of the process. Overall, this approach is well developed, though innovations to increase efficiency and lower costs are still needed in order to bring the cost of hydrogen production with this method closer to that of hydrogen production from other sources .However, hydrogen is not expected to be available before the year 2020, although demonstration fleets are announced to be in operation earlier. The biggest obstacle for an earlier large-scale implementation is the missing environmental friendly, and economically and technically mature developed source for hydrogen production.
As cited in ‘An EU Strategy for Biofuels’, “advanced biofuel technologies could also provide a stepping stone to renewably-produced hydrogen, which offers the prospect of virtually emission-free transport. However, hydrogen fuel cells require new engine technology as well as a big investment in plants to produce the hydrogen and a new distribution system. In this context, the sustainability of hydrogen has to be carefully assessed. Any shift to hydrogen-based transport would therefore call for a major decision, embedded in a large-scale, long-term strategy” (EC 2006b p. 29).
In particular, energy effective use of hydrogen requires the introduction of fuel cells instead of internal combustion engines and therefore, adds another technology and cost challenge. The implementation of fuel cell vehicles is promising a much higher TTW efficiency than hydrogen combustion engines. Hydrogen from renewable sources for fuel cell driven vehicles might be a long term option, but its introduction will take a long time, needs breakthroughs in technology and cost and will require intermediate steps to enable a gradual growth of both fuel and vehicle availability.
The use and logistics of hydrogen becomes a difficult problem, since hydrogen in its gaseous state takes up a very large volume when compared to other fuels. One possible solution is to use ethanol to transport the hydrogen, then liberate the hydrogen from its associated carbon in a hydrogen reformer and feed the hydrogen into a fuel cell. Alternatively, some fuel cells (DEFC Direct-ethanol fuel cell) can be directly fed by ethanol or methanol.
Experiences with infrastructure of hydrogen applications have been made in several countries. For example in Germany a hydrogen filling station opened at Munich Airport26 in May 1999.
In April 2003 the first hydrogen refueling station was opened in Reykjavík, Iceland. This station serves three buses that are in service in the public transport net of Reykjavík. The station produces the hydrogen it needs by itself, with an electrolyzing unit, and does not need refilling: all that enters is electricity and water.
Probably the most prominent worldwide project of hydrogen application for transport purposes is the establishment of the hydrogen highway in California, USA, promising 100 hydrogen fuel stations and 2000 hydrogen vehicles till the end of 2010. Other regions, as for example British Columbia (Canada) and Norway have joint the idea of the “hydrogen highway”.
References
- COSTA GOMEZ C. (2006): Notwendige Rahmenbedingungen für die Nutzung von Biogas als Kraftstoff in Deutschland. Presentation at the „4. Internationaler Fachkongress: Kraftstoffe der Zukunft des BBE und der UFOP“; 27./28. November 2006, ICC Berlin
- CRUTZEN P.J., MOSIER A.R., SMITH K.A., WINIWARTER W. (2007): N2O release from agro-biofuel production negates global warming reduction by replacing fossil fuel. – Atmos. Chem. Phys. Discuss., 7, 11191-11205
- DREIER T., TZSCHEUTSCHLER P. (2001): Ganzheitliche Systemanalyse für die Erzeugung und Anwendung von Biodiesel und Naturdiesel im Verkehrssektor. - Energie und Anwendungstechnik; Insttitut für Energeietechnik TU München; 70 p.
- EC (COMMISSION OF THE EUROPEAN COMMUNITIES): Directive 2001/116/EC of 20 December 2001 adapting to technical progress Council Directive 70/156/EEC on the approximation of the laws of the Member States relating to the type-approval of motor vehicles and their trailers. – OJ L 18, 115 p.